Contamination by halogenated volatile organic compounds (VOCs) may be considered to be a widespread problem at U.S. Department of Energy (DOE) and military sites. It also has environmental ramifications. Compounds such as carbon tetrachloride, trichloroethylene, tetrachloroethylene, etc. may commonly be referred to as dense nonaqueous phase liquids (DNAPLs). These compounds may have been used extensively in degreasing and equipment cleaning operations in the past, with disposal practices that led to their release into the ground, and thus may be considered the most significant organic contaminants in groundwater associated with disposal sites (Plumb 1992).
For measurement of concentrations or amounts, the photoionizaton detector (PID) may be among the most common VOC field measurement tool in use today. A typical PID lamp energy may be 10.6 electron volts (eV), which can be sufficient for ionizing compounds containing double bonds. However, halogenated compounds without double bonds such as carbon tetrachloride or methylene chloride may require an energy of 11.7 eV for ionization (Table 5) (Schabron et al. 1996). This may only be accomplished with a PID equipped with a lithium fluoride window, which may be considered to have a short lifetime due to the solubility of lithium fluoride in water. Also, a PID may not be considered to be selective for halogenated compounds. Many other compound types may be detected also. Field screening of soils with a PID probe may involve placing a soil sample in a plastic bag or a glass jar, sealing the bag or covering the jar with aluminum foil, then inserting the PID probe tip through the foil (Hewitt and Lukash 1997).
In an unrelated field, leak testing of refrigerants is often conducted in situations warranting isolated testing events. In such situations, heated diode and corona discharge sensors are used merely as alarm sensors to detect leaks of refrigerants from air conditioners, freezers, and refrigerators, since both heated diode and corona discharge sensors are selective to the presence of halogens or carbon-halogen bonds. These test procedures, however, have been viewed as not applicable to quantitative analysis.
In situations calling for quantitative analysis of VOCs, PID's are used. Besides the aforementioned problems, though, such hand-held PID detectors may also suffer from the disadvantage in that they may not be able to discriminate between halogenated and non-halogenated species (Table 5). A more detailed analysis which may also allow for some speciation involves a portable gas chromatograph (Myers et al. 1995, Linenberg 1995). This is a relatively expensive type of device, however, skilled operators are usually required, as is the flow of a chemically inert carrier gas. Immunoassay kits may also allow for rapid field analysis (Hudak et al. 1995). This approach may require temperature control and critical timing for the several steps involved.
Several novel approaches have been proposed for surface or down-hole screening of halogenated VOCs in the field. One approach may use refractive index attenuation on coated optical fibers (Le Goullon and Goswami 1990). Another technology may utilize a chemical reaction in a basic media to form a color in the presence of trichloroethylene (Rossabi et al. 1993). Yet, another probe may use a heated LaF2 doped element heated to 600° C. to measure volatile chlorine containing compounds (Buttner et al. 1995). A synthetic nose consisting of an array of different chemicals which may give different optical response to various volatile analytes has been proposed (Walt 1998). Other approaches may also include Raman spectroscopy (Ewing et al. 1995, Haas et al. 1995), electrochemical cells (Adams et al. 1997), acoustic wave devices (Frye et al. 1995), and ion mobility spectrometry (Stach et al. 1995). The above devices certainly may all contribute some progress towards the problem of monitoring for some of the VOC indicator compounds at various levels, but none meet user needs across the full spectrum.
Thus, there exists a need for a new type of simple field monitor (such as a portable field kit) which is selective to halogenated VOCs, field-worthy (portable, not overly complex to operate, not requiring extensive (or perhaps any) on-site lab facilities, and enabling field monitoring, including in situ sensing or operating of the monitor), and does not require skilled operation (meaning it is easy to operate by anyone with only minimal instruction). HVC (halogenated volatile compound) sensing (or more generally sense operating) upon merely positioning a sensor in an area of interest an activating a switch(es) is a desirable feature. Sensing as used herein may refer to sensing for the presence of a chemical and/or determining the concentration of a chemical. Monitoring may be characterized by any type of chemical group (halogenated VC, e.g), or by purpose (environmental, groundwater, or soil, as but a few examples). Monitoring includes sensing to assess the presence of a chemical functional group and/or sensing to determine the concentration of a chemical functional group.
The presence of chemical warfare agents (CWAs) and terrorist substances is one of increasing concern. Hand-held and portable sensor systems are commercially available for the detection of various chemicals in vapor form in ambient air. These sensors typically use a detector system based on photoionization, corona-discharge, heated diode, thermal conductivity, ion mobility spectrometry, ion capture, or other technology. Many of the sensor systems incorporate an air pump to flow sampled air past the detector, while other sensors use air diffusion. Generally, they do not, however, offer adequate differentiation to be employed in the highly sensitive security setting. For example, if only a halogen-selective detector device were used for the detection of halogen-containing chemical warfare agents (CWAs) such as sarin, soman, phosgene, and sulfur mustard, the inability to differentiate between these chemicals and other more common chemicals such as refrigerants, dry cleaning solvents, and degreasing solvents might cause a false alarm in a public setting. This could cause panic and hysteria.
Current state-of-the-art sensor technology suffers to some degree from a lack of applicability of individual sensors to a variety of chemical vapors. Often, individual detector systems are either too specific or too broad in scope for measuring a suite of chemical vapors. A detector that is too specific, such as corona-discharge, is limited to detection of a unique chemical structure and cannot evaluate chemicals of different classes. A detector that is too broad-based, such as thermal conductivity, responds to almost any vapor without regards to chemical specificity. Systems capable of identifying individual compounds, such as mass spectrometers, can often be too expensive for widespread deployment.